PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 21.3	$ 19.6
Interest Cost	32.6	44.3
Expected Return on Plan Assets	(108.9)	(100.3)
Actuarial Loss	61.6	49.3
Prior Service Cost/(Credit)	0.3	0.3
Total Net Periodic Benefit Expense/(Income)	6.9	13.2
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.5	2.6
Interest Cost	4.4	6.1
Expected Return on Plan Assets	(19.7)	(18.1)
Actuarial Loss	2.6	2.1
Prior Service Cost/(Credit)	(5.3)	(5.4)
Total Net Periodic Benefit Expense/(Income)	(14.5)	(12.7)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	6.3	5.7
Interest Cost	7.3	9.5
Expected Return on Plan Assets	(21.6)	(20.0)
Actuarial Loss	13.0	10.0
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	5.0	5.2
Intercompany Allocations	1.3	1.9
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.6	0.5
Interest Cost	0.8	1.2
Expected Return on Plan Assets	(2.6)	(2.4)
Actuarial Loss	0.4	0.4
Prior Service Cost/(Credit)	0.2	0.2
Total Net Periodic Benefit Expense/(Income)	(0.6)	(0.1)
Intercompany Allocations	(0.3)	(0.4)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.0	3.8
Interest Cost	6.7	9.6
Expected Return on Plan Assets	(26.9)	(25.8)
Actuarial Loss	15.5	13.3
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Expense/(Income)	(0.6)	1.0
Intercompany Allocations	1.5	1.9
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.6	0.6
Interest Cost	1.1	1.7
Expected Return on Plan Assets	(9.3)	(8.5)
Actuarial Loss	0.6	0.7
Prior Service Cost/(Credit)	(4.2)	(4.2)
Total Net Periodic Benefit Expense/(Income)	(11.2)	(9.7)
Intercompany Allocations	(0.4)	(0.4)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.2	2.2
Interest Cost	3.6	4.9
Expected Return on Plan Assets	(11.9)	(11.3)
Actuarial Loss	4.9	4.1
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	(1.2)	(0.1)
Intercompany Allocations	0.5	0.6
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.2
Interest Cost	0.5	0.6
Expected Return on Plan Assets	(1.5)	(1.4)
Actuarial Loss	0.3	0.1
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Expense/(Income)	(0.3)	(0.4)
Intercompany Allocations	$ (0.1)	$ (0.2)